1. Field of the Invention
The present invention relates in general to a vitrified abrasive solid mass such as a vitrified grinding wheel, grindstone or honing stick which has a vitrified abrasive structure impregnated with a cured composition comprising resin and solid lubricant agent.
2. Discussion of the Related Art
There is known a vitrified abrasive solid mass, having a vitrified abrasive structure, in which abrasive grains and an optionally used aggregate are held together by a glassy inorganic vitrified bonding agent, and in which a multiplicity of permeable pores or voids are formed between the abrasive grains. Owing to excellent properties of the vitrified abrasive solid mass, such as a relatively high degree of strength with which the bonding agent holds the abrasive grains together, and a relatively easy dressing operation, the vitrified abrasive solid mass is widely used for precision grinding operations. In recent years, there is a need for a vitrified abrasive solid mass capable of performing a grinding operation with improved efficiency, to meet a demand for shortening of the required grinding time.
On the other hand for practical use, there has been proposed a vitrified abrasive solid mass wherein the proportion or content of the inorganic bonding agent is made relatively high for increasing a grade of the vitrified abrasive solid mass and also a degree of strength with which the inorganic bonding agent holds the abrasive grains together. This vitrified abrasive solid mass has a relative high hardness, and can meet, to some extent, the above-indicated need for improving the grinding efficiency. However, this vitrified abrasive solid mass is not completely satisfactory. Namely, the use of the inorganic bonding agent in a relatively large proportion assures an increase in the strength of the vitrified abrasive solid mass, which contributes to an improvement in the grinding efficiency, but considerably reduces the porosity of the vitrified abrasive structure and results in difficult or insufficient fracturing and removal of the abrasive grains, leading to relatively easy glazing, clogging or fusion of the surface of the vitrified abrasive solid mass, relatively easy chipping of the abrasive structure, relatively difficult dressing operation of the vitrified abrasive solid mass, and other drawbacks encountered during use of the vitrified abrasive solid mass as a grinding tool. In addition, the use of the inorganic bonding agent in the relatively large proportion is likely to cause various drawbacks during the manufacture of the vitrified abrasive solid mass, such as cracking or deformation of the abrasive solid mass and insufficient removal or burn-out of the primary binder of the inorganic bonding agent in the firing process. The insufficient removal of the binder may cause the manufactured vitrified abrasive solid mass to have some amount of residual carbon.
Such vitrified abrasive solid mass with reduced porosity may be manufactured by heat-pressing of the materials of the grindstone, and may be used for performing a highly efficient grinding operation. However, this manner of manufacturing the vitrified grindstone requires a special manufacturing equipment which usually suffers from low manufacturing efficiency, leading to a relatively high cost of manufacture of the vitrified abrasive solid mass, and a considerable dimensional limitation of the vitrified abrasive solid mass that can be manufactured.
It is therefore a first object of the present invention to provide a vitrified abrasive solid mass which has a vitrified abrasive structure impregnated and cured with a composition comprising resin and solid lubricant agent and which is less likely to suffer from the above-indicated drawbacks such as easy glazing or clogging, while maintaining characteristics relating to its use, the easy dressing and also with a high degree of grinding efficiency.
This object may be achieved according to this invention, which provides an abrasive vitreous bonded solid mass having a vitrified abrasive structure comprising abrasive grains which are held together by an inorganic bonding agent, wherein the abrasive structure is impregnated with a cured composition which comprises a resin and solid lubricant agent.
In the vitrified abrasive structure of the present vitrified abrasive mass, a suitable volumetric percentage of the total volume of the pores, or voids is filled with the cured composition comprising resin and the solid lubricant agent, so as to prevent filling of the pores with used abrasive grains or metal chip particles which are removed from the work-piece during grinding operation using the present abrasive vitrified solid mass and which would otherwise be fused in the pores, causing clogging or glazing on the grinding surface of the vitrified abrasive solid mass. It is also noted that since the said composition filling the pores is softer than the abrasive grains, the surface of the vitrified abrasive mass is comparatively recessed at local spots corresponding to the pores filled with said composition, during the grinding operation on the work-piece, so that the abrasive grains adjacent to the surface of the abrasive structure can gradually fracture or break down and are removed, making it possible to prevent an excessive rise of the temperature on the work-piece surface caused by an excessive amount of frictional heat which would have been generated between the surface work-piece and the remaining dull abrasive grains, and also can prevent chipping of the vitrified abrasive mass while permitting easy dressing of the abrasive vitreous solid mass. The abrasive grains which are only loosely held together by the inorganic bonding agent can be reinforced with an additional bonding force provided by the cured composition comprising resin and solid lubricant agent, so that the cured resin functions to avoid early removal of those abrasive grains assuring a high grinding ratio.
It is further appreciated that the solid lubricant agent in said composition enables to replace the lubricant grinding fluids, which are used in the grinding operation, with city water or cooling air wind.
It is to be understood that the vitrified abrasive structure of the abrasive vitrified solid mass may further include such aggregates that are held together with the abrasive grains by the inorganic bonding agent.
According to a first preferred form of the invention, in the cured composition, the resin consists of at least one thermosetting synthetic resins which have been selected from a phenol resin or an epoxy resin, so that the vitrified abrasive solid mass has a higher degree of hardness than the portion of the cured composition.
According to a second preferred form of the invention, the abrasive grains include super abrasive grains (considerably fine abrasive grains) consisting of diamond abrasive grains, CBN abrasive grains, or mixture of diamond and CBN abrasive grains. The super abrasive grains preferably have Knoop hardness of at least 3000. It is also preferable that the super abrasive grains have an average particle diameter size of 20-220 xcexcm. The particle sizes of 20 xcexcm and 220 xcexcm diameter correspond to 800 and 60 meshes per linear inch, respectively. Preferably, the super abrasive grains in the vitrified abrasive structure have a concentration of larger than 10 and smaller than 230, more preferably, a concentration ranging from 20 to 200.
According to a third preferred form of the invention, the vitrified abrasive structure has a porosity of 20-75% by volume, more preferably, 30-65% by volume, before the vitrified abrasive structure is impregnated with the resin.
According to a fourth preferred form of the invention the inorganic bonding agent consists of a borosilicate glass or crystallized glass which is suitable for holding super abrasive grains together. The crystallized glass may be, for example, a glass in which willemite(Zn2 SiO4) precipitates. The inorganic bonding agent preferably has a thermal expansion coefficient ranging from xcex1xe2x88x92(2xc3x9710xe2x88x926) to xcex1+(2xc3x9710xe2x88x926) [1/K] at a temperature from room temperature to 500 degree C. (where xcex1 represents a thermal expansion coefficient of the super abrasive grains), so that the super abrasive grains can be tightly bonded together by the inorganic bonding agent.
According to a fifth preferred form of the invention, the vitrified abrasive structure includes 15-35% by volume of the inorganic bonding agent, so that the porosity of the vitrified abrasive structure is held in the volumetric range as described above, without deteriorating the holding strength with which the bonding agent holds the abrasive grains together. The vitrified abrasive structure may include, as an aggregate, a pore forming agent such as an inorganic ballooning agent or other inorganic hollow substance.
According to a sixth preferred form of the invention, the solid lubricant agent comprises hexagonal crystal boron nitride, molybdenum disulfide, black carbon or graphite, which are sufficiently smaller diameter than pore diameter of vitrified abrasive mass, for example, an average particle diameter of 1 xcexcm or less. Thus, the solid lubricant agent can be evenly arranged in the structure of the vitrified abrasive solid mass so as to obtain the preferable lubricant effect.
According to the seventh preferred form of the invention, the solid lubricant agent in the composition to be impregnated into the vitrified structure is in the range from 1% to 80% by weight against the resin. If the percentage by weight is less than 1%, it is difficult to obtain the sufficient lubricant effect, and as for more than 80% the insufficient impregnation to the vitrified structure will result.